There’s a ton of talk about intermittent fasting in the ancestral heath sphere for general health and wellness as well as weight loss, but little indication of specific applications for the practice. Anytime you attempt a “radical” health practice like not eating, it helps to have a good reason to do it. That will not only give you something to aim for, but it will ensure you actually have a physiological justification for your experiment. Never go in blind.
What are some of the specific scenarios and conditions where fasting makes the most sense?
1. You Are Intractably, Morbidly Obese
It used to be that an accepted and well-tested fix for morbid obesity that was unresponsive to other methods was long term fasting.
One experiment was very long term: over a year of not eating anything except for multivitamins. (Disclaimer: I’m not recommending this approach, but it is interesting.)
Back in 1965, an obese Scotsman of 27 years and 456 pounds came to the Department of Medicine in Dundee, Scotland, with a problem. He needed to lose weight. A (1/8 of a) ton of it. The doctors suggested maybe not eating for a few days could help. It was just an offhand recommendation, but the Scotsman really took to it. He stayed at the hospital for several days, taking only water and vitamin pills while undergoing observation to ensure nothing went wrong. When his time was up, he continued the fast back at home, returning to the hospital only for regular monitoring. After a week, he was down five pounds and feeling good. His vitals checked out, blood pressure was normal, and though he had lower blood sugar than most men, he didn’t seem particularly impaired by it. The experiment continued… for 382 days.
Yes, AB fasted for 382 days, drinking only water and taking vitamin, potassium, and sodium supplements. All told, he lost 276 pounds, reaching his target weight of 180 pounds and maintaining the bulk of his weight loss. Over the five following years of observation, AB regained just sixteen pounds, putting him in excellent, but underpopulated territory (at least 80% of dieters eventually regain all the lost weight).
2. You Want the Benefits Of Ketosis Without Having To “Go Keto”
One thing a fast of sufficient length will do is throw you straight into ketosis. Humans are so wired to go into ketosis that a simple overnight “fast,” aka sleeping, will do it.
Then, when you do eat, you have more wiggle room on carbs because you’ve just spent plenty of time in ketosis during the fast. This isn’t the same thing as going keto, but then again, not everyone wants to be in ketosis all the time. Many benefits come from “dipping in and out of ketosis” on a regular basis, and regular intermittent fasting certainly qualifies.
3. You’re Otherwise Quite Lean, Active, and Low-Stress and Just Have a Little Bit To Lose
Fasting can be a stressor. Going without food tends to do that in organisms that rely on food for sustenance. It’s just that in the context of an overall low-stress lifestyle and low-oxidative stress physiology, it can be a positive stressor—a stressor that promotes strength and adaptation.
This is why women, in general, tend to have a tougher time with long term fasting. They are inherently more vulnerable to nutritional stressors since they have to be prepared to carry children to term and nurse them, two functions that require a steady source of calories. Biologically speaking, that is.
4. You Want a Buffer Against Degenerative Diseases
Now, this is mostly speculative. This isn’t medical advice or a guarantee of any kind. There’s good reason to believe that regular extended fasting (or at least skipping meals/multiple meals on a regular basis) can reduce the risk of degenerative diseases and perhaps even extend life by triggering the autophagy pathway that cleans up damaged cells and keeps pre-cancerous cells suppressed.
Will this ensure you don’t get cancer down the line or die earlier than is your potential? No, not at all. But it’s a relatively easy thing to try with no downside, and it just might help.
5. You Want To Lean Out and Gain Muscle At the Same Time
The classic Leangains-style intermittent fasting with regular strength training is one of the best ways I’ve ever found to gain muscle and lose body fat concurrently. You follow a shortened eating window every day—usually 16 hours fasting, 8 hours eating—and on workout days end the fast with a strength training workout, then eat. Classic Leangains has you eating lower fat, higher carb on workout days and higher fat, lower carb on rest days, with protein kept high throughout. But it should work on whatever macro combination you prefer.
You won’t gain as much muscle as quickly as if you ate enormous meals all the time, but the gains you make will generally be leaner.
6. You’re Recovering From Major Gut Issues
A friend of mine just did a 5-day water fast to reset his gut biome after SIBO and/or a parasitic invasion. It fixed him right up. And whenever my dogs have ever had digestive upset, like diarrhea or something, I’ll throw them on a two-day fast and they bounce right back.
I think the gut needs periodic “resets” to stay in top shape. Give it a rest, have nothing go through demanding its attention for a couple days, and allow things to balance out. Just like someone who trains all the time can really benefit from a deload week, a digestive system that’s constantly digesting and processing food can benefit from a day or two of rest.
When I travel for business, which is quite often, I tend to fast. Airports are getting better, but it’s still a sad state of culinary affairs. I usually have a few choices: I can pick at a wilted Caesar salad with flaccid chicken breast. I can eat some congealed beef patties from whatever fast food joint has set up shop in the terminal. I can drop $30 for a mediocre steak. Or I can just fast.
I usually choose the last option. At this point in my life, I refuse to put substandard food into my body, especially if it isn’t even very delicious. I’d rather just skip the food entirely and have a great meal when I arrive.
9. You Can’t Stop Snacking
Total freedom is hard for some people to manage. Even if the food is high quality and Primal or keto or whatever, constant access to eternal amounts of it is hard to turn down. Snacking happens. Again and again. Sometimes, we need to put up barriers to manage that freedom, to make it work. After all, paradise is a walled garden, and erecting the artificial eating barrier of a full-on fasting day (or two) or a compressed eating window will allow you to overcome this. If this describes you, a fasting regimen just might be the trick to work.
Plus, many people find that forcing yourself to not eat for an extended amount of time on a regular basis upregulates fat burning machinery and allows better eating habits and reduced snacking when you do go back to normal eating.
10. You’re Willing To Try an Unconventional Recovery Technique
When Dude Spellings was on the podcast, he relayed a wild story about racing 50 miles through the Grand Canyon in a (mostly) fasted state, being greeted at the finish line with a stack of pizzas, and instead of wolfing down with all the other competitors, continuing the fast through till the next day—theorizing that in his exhausted, inflamed state he could use the benefits of cell repair and anti-inflammatory processes enhanced by fasting. He woke feeling less stiff and sore than his previous crossing 13 years prior.
11. You’re Trying To Avoid Jet Lag
Another reason I often fast when traveling is to establish a new circadian rhythm aligned with my destination. By waiting until the morning after my arrival to eat, I take advantage of one of the most powerful stimuli, or zeitgebers, for establishing a new circadian rhythm: food. Eat a big meal in the morning, and your body “knows” it’s morning—biologically speaking.
How this looks:
I arrive at noon in the new location, which feels like nighttime for me. Instead of eating a big “lunch” and collapsing into bed, I spend all day staying active and fasting. I skip dinner. I walk everywhere. Then, in the morning, I get a workout in, preferably outdoors to get natural light exposure, and follow up with a big breakfast. That combo—the light, the workout, and the breakfast eaten at my desired breakfast time in the new place—sets my internal clock and minimizes jet lag.
12. You’re a Shift Worker
Shift workers are at an increased risk of many diseases, like diabetes and breast cancer, and a lot of this comes down to the disordered eating they’re often forced to engage in. They eat in the middle of the night, when their body wants to be sleeping, and in doing so throw their circadian rhythm out of wack more than it already is going to be.
If you’re primarily awake in the middle of the night but want to maintain a semblance of circadian rhythm, it makes sense to eat at normal dinner time and at the end of your shift, but not during. Fasting during your shift might just be the big breakthrough.
13. You’re Undergoing Chemotherapy
It’s common knowledge that calorie restriction can improve the response to chemotherapy while reducing the negatives. Fasting is just a more reliable, arguably easier version of calorie restriction. There’s even evidence that fasting can improve your healthy cells’ resistance to chemotherapy while reducing the cancer cells’ resistance while reducing negative symptoms like nausea and vomiting.
If you have a history of eating disorders, this is probably unwise. The feast/fast method can be taken to unhealthy levels, especially if it’s couched in feelings of body dissatisfaction or deep childhood trauma. But if these aren’t an issue and you have a one-off feast (like a holiday dinner) you simply want to really dig into, fasting for a day before the big feast can enhance the effects of the feast.
Whenever I hit the Brazilian all-you-can-eat BBQ joint, I’ll fast for at least a day—just to get my money’s worth and really develop that insatiable, salivating, Primal urge to eat meat. Hunger is the best spice.
These aren’t even all the scenarios where fasting helps or makes sense. There are others, which is where you come in. What have been your reasons for fasting? Has it worked?
Thanks for reading, everyone. Take care!
About the Author
Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.